Polycyclic aromatic hydrocarbons and polychlorinated biphenyls in forest soils: depth distribution as indicator of different fate.

We determined 20 polycyclic aromatic hydrocarbons (PAHs) and 12 polychlorinated biphenyls (PCBs) in Oi, Oe, and Oa horizons and at 0-5 and 15-20 cm mineral soil depth of 16 Norway spruce (Picea abies) stands in north Bavaria. The sum of PAH concentrations increased along the line Oi (mean+/-SD: 841+/-330 microg kg(-1))<Oe (4117+/-3093)<Oa (9272+/-5721) while that of PCBs was higher in Oe (39.1+/-19.4 microg kg(-1)) than in Oi (12.1+/-6.5) but similar in Oe and Oa (46.0+/-39.4). The sum of both PAH (0-5 cm: 666+/-707/15-20 cm: 95+/-165) and PCB concentrations (1.7+/-1.5/0.28+/-0.20) decreased sharply in the mineral soil. The concentration ratios of an individual compound in the Oe to that in the Oi (enrichment factor, e(Oe/Oi)) and the analogously calculated e(Oa/Oe) correlated significantly (p<0.001) with the octanol-water partition coefficient (K(ow), r=0.94 and 0.93 for PAHs; r=0.91 and 0.83 for PCBs, respectively). The enrichment factors of PAHs were consistently higher than those of PCBs with comparable K(ow). This may be explained by a higher volatilisation rate of the PCBs or a stronger sorption of PAHs to soil. In the mineral soil, the concentrations of most PAHs and PCBs decreased with increasing depth, except those of naphthalene and the PCBs 8, 20, 28, and 52. In the mineral soil, the enrichment factors e(0-5/Oa) and e(15-20/0-5) of the PCBs 8, 20, 28, and 52 were, on average, 10 times higher than those of PAHs with similar K(ow) values indicating a more pronounced leaching of PCBs than of PAHs. The results demonstrate that the fate of PAHs in forest soils is markedly different from that of PCBs.